A sensor that measures physical quantities such as temperature and strain using an optical fiber have some advantages such as a long operating life, a lightweight, a thin diameter, and a flexibility, thereby enabling it be used in narrow spaces. In addition, this sensor has a characteristic of a strong resistance to electromagnetic noise due to insulation property of the optical fiber. It is therefore anticipated that this sensor will be used for structural health monitoring of large constructions such as bridges and buildings, and aerospace equipment such as passenger airplanes and manmade satellites.
Performance requirements of the sensor for applying the structural health monitoring in these structures include high spatial resolution, and having a multipoint (multiplexed) sensor (having a wide detection range), and a capability of real time measurement, and the like.
While various optical fiber sensor systems have already been proposed, an optical fiber sensor using an FBG sensor and OFDR analysis method is a most promising optical fiber sensor that fully satisfies the above performance requirements.
The optical fiber sensor system using the FBG sensor and the OFDR analysis method determines the position of the FBG sensor by using cyclic change in interference light intensity between Bragg reflected light from the FBG sensor and reflected light from a referential reflecting end. In addition, this optical fiber sensor system measures strain and temperature of a detection part based on an amount of change in the wavelength of the Bragg reflected light.
Hitherto disclosed examples of this optical fiber sensor system include one with high spatial resolution of 1 mm or less (e.g. see Non-Patent Literature 1), one in which eight hundred FBG sensors are multiplexed on an eight-meter optical fiber, and one can measure strain at more than three thousand points with a total of four optical fibers simultaneously (e.g. see Non-Patent Literature 2), and one can real time measurements (e.g. see Patent Literature 1). In addition, according to Non-Patent Literature 1, it is also possible to measure strain distribution along the long direction of the FBG sensors (“strain distribution” signifies that the amount of strain along the long direction of the FBG sensors is uneven). Patent Literature 3 also describes means for measuring of strain distribution.
A general problem of optical fiber sensor systems includes that, when there is change in a plurality of physical quantities such as temperature and strain, it is not possible to independently identify and measure amount of these changes. Consequently, for example, when using the optical fiber sensor system as a strain sensor, a separate temperature-compensating sensor must be used so that temperature change of a detection part is not treated as the change in strain.
To solve this problem, a method using FBG sensors consist of PM fibers has been proposed (e.g. see Patent Literature 2). In this method, PANDA type PM fiber is used for FBG sensor, and temperature and strain can be measured simultaneously by measuring the amount of change in the wavelength of Bragg reflected lights from two orthogonal polarization axes at the FBG sensor consists of this PANDA fiber.
That is, this method provides a strain sensor that does not require a temperature-compensating sensor.
Conceivably, if the technologies mentioned above are combined in an optical fiber sensor system using FBG sensors consist of PM fiber and OFDR analysis method; it will be possible to achieve high spatial resolution, multipoint measuring, real time measuring, and simultaneous measurement of temperature and strain.
[Patent Literature 1] Japanese Patent No. 3740500
[Patent Literature 2] Japanese Patent No. 3819119
[Patent Literature 3] Japanese Patent No. 4102291
[Non-Patent Literature 1] H. Murayama, H. Igawa, K. Kageyama, K. Ohta, I. Ohsawa, K. Uzawa, M. Kanai, T. Kasai and I. Yamaguchi, “Distributed Strain Measurement with High Spatial Resolution Using Fiber Bragg Gratings and Optical Frequency Domain Reflectometry” Proceedings OFS-18, ThE40 (2006)
[Non-Patent Literature 2] B. Childers, M. E. Froggatt, S. G. Allison, T. C. Moore, D. A. Hare, C. F. Batten and D. C. Jegley, “Use of 3000 Bragg grating strain sensors distributed on four eight-meter optical fibers during static load test of a composite structure.” Proceedings SPIE's 8th International Symposium on Smart Structure and Materials, Vol. 4332, pp. 133-142 (2001)